Girl, 22yo - 5'2'', 8st backpack advice!

I am planning a trip round Asia for six months, leaving in February. I have been reading up on backpacks (rucksacks?) but am finding the advice (in the nature of advice, I suppose) contradictory.

I am strong for my size but on the other hand will be packing very lightly, as I am aware lots of what I'll need will be readily available. We are planning to spend about a month each in India, Nepal, Thailand, Laos, Cambodia and Vietnam. We hope to fly as little as possible - except to India and home from Thailand but we are aware we might need to fly over Burma.

I have been trying to research which backpack would be right for me but feel the need to ask some advice - I have been recommended sizes between 40l and 90l! What is the correct size for this kind of trip? And which brands do people find durable but not cripplingly expensive?

I would recommend to take a 40l (max), this will leave even place for souvenirs or other stuff you'll buy along the way.I travel with a bag of around 35l, which weights 8-9 kg, and a bag like this is very handy on busrides, to take a rickshaw, to look for a hostel,...

OK so have now done more reading, and visited a couple of outdoor stores. What I learned there was that the store assistants didn't seem to know anything about rucksacks and that the range was awful. I tried on a Eurohike Pathfinder 35 (probably too small?) and a couple of others but they didn't seem to have any 40l packs.